1. Field of the Invention
The present invention relates to an optically active device which comprises an amplification optical fiber doped with an optically active material.
2. Related Background Art
Examples of optically active devices which comprise amplification optical fibers doped with optically active materials include fiber laser light sources and optical fiber amplifiers, which have been widespread in various uses such as processing in electronic/mechanical fields, medicine, measurement, and optical communications. In uses requiring high-power laser light such as laser processing in particular, unnecessary energy becomes heat by the photon energy difference between pumping light and output laser light. From the viewpoint of lowering this thermal energy, it has been desired that the pumping light and output laser light have respective wavelengths close to each other. Therefore, the use of ytterbium (Yb), which is one of rare-earth elements, as an optically active material added to amplification optical fibers has been increasing.
As shown in FIG. 8, a Yb-doped fiber (YbDF) usually has an output laser light wavelength in the wavelength region of 1030 nm to 1100 nm where Yb ions have a gain because of absorption and induced emission characteristics of Yb. On the other hand, the band of 0.92 μm (0.92 to 0.93 μm) and the band of 0.98 μm (0.97 to 0.98 μm) have typically been in use as the pumping light wavelength. From the above-mentioned viewpoint of making the respective wavelengths of output laser light and pumping light closer to each other, the band of 0.98 μm is desirable as the pumping light wavelength in particular. FIG. 8 is a graph showing the wavelength dependency of normalized unsaturated absorption coefficient concerning various samples of Yb-doped fiber (YbDF 41 to 43). The samples YbDF 41 to 43 have various characteristics as shown in FIG. 3. Namely, the sample YbDF 41 has an unsaturated absorption peak of 250 dB/m, a core diameter of 2.4 μm, and a cladding diameter of 125 μm. The sample YbDF 42 has an unsaturated absorption peak of 180 dB/m, a core diameter of 4.0 μm, and a cladding diameter of 125 μm. The sample YbDF 43 has a double cladding structure with an unsaturated absorption peak of 9 dB/m, a core diameter of 15.0 μm, and an inner cladding diameter of 125 μm.
However, a multimode-pumping laser diode (LD) is used as a pumping light source in the cladding pumping scheme often employed in uses requiring high-power laser light. In this case, the pumping power reaches several watts, whereby the temperature of an LD chip is hard to adjust. As a result, there is a possibility of the pumping light wavelength changing because of fluctuations in temperature of the LD chip. In the absorption spectrum of Yb, the band of 0.98 μm has a sharp peak, whereby the pumping light wavelength may deviate from this absorption peak depending on changes therein. This means the occurrence of a state where the pumping light is hardly absorbed by Yb but is transmitted through the Yb-doped optical fiber (YbDF). From such a viewpoint, it is desirable that the band of 0.92 μm be used as the pumping light wavelength in cladding pumping.
In the core pumping scheme employing a single-mode pumping LD, on the other hand, it is hard to neglect the present state where 0.98-μm-band-pumping LD modules incorporating temperature-adjusting devices are widely available in the market for communications so that there is no obstacle for pumping in the band of 0.98 μm.